I. Field of the Invention
The invention generally relates to wireless telephone systems and in particular to public payphones connected to wireless telephone systems.
II. Description of the Related Art
Wireless telephone systems including, for example, cellular telephone systems and wireless local loop systems, have become quite popular for private, individual use. Recently, wireless telephone systems are being extended to include public payphones. Wireless payphones may be configured like payphones connected to conventional land lines, such as the public switched telephone network (PSTN), but the wireless payphone transmits telephone call signals to a remote base station rather than directly onto hardwired land lines. The remote system may include, for example, space based satellites, cellular base stations, or the like. Wireless payphones are particularly desirable for use within areas that do not include conventional land lines, including remote portions of the United States as well as portions of developing countries. Wireless payphones may also be advantageously employed even within areas having conventional land lines to allow consumers a choice between the telephone company operating the land lines and telephone companies offering wireless service in the area. Wireless payphones are also ideal for use within commercial airliners, trains, buses, ships, or the like, wherein conventional land lines are not available.
However, problems arise in implementing wireless payphones insofar as metering the charges incurred by the user of the payphone. With a conventional payphone connected to a land line, metering is typically achieved by transmitting metering pulses to the payphone from the network. The metering pulses are typically brief tones set at, for example, 4,000 hertz (Hz). The payphone may include a filter for filtering out the tone so that the user does not hear the tones during the telephone call. Usually, the payphone is preprogrammed with some monetary value associated with each metering pulse to allow the actual charges incurred during a telephone call to be calculated from the total number of metering pulses received during the telephone call. Each individual metering pulse may represent, for example, one or two cents worth of telephone calling time. During peak hours, when telephone calling rates are at their highest, the metering pulses are transmitted fairly frequently. During off hours, when calling rates are lower, the metering pulses are transmitted less frequently.
Typically, the user is required to insert some initial quantity of money into the payphone to initiate a telephone call. Thereafter, while the telephone call is in progress, metering pulses are received by the payphone and the amount of money associated with each metering pulse is deducted from the initial amount of money inserted. A display may be provided to indicate to the user the amount of money remaining. Also, typically, as the amount of money remaining decreases below some minimum threshold, perhaps one dollar, a warning message or the like is provided to the user indicating that the telephone call may be automatically terminated if additional money is not inserted into the payphone. In many state of the art payphones, the user is not limited to inserting cash only. Various types of debit cards may alternatively be employed wherein the debit card is preprogrammed with some quantity of money. Alternatively, the charges of the telephone call may be billed to the receiving party or perhaps to a credit card account, calling card account or the like. Indeed, many state of the art payphones do not accept cash whatsoever but operate only using debit cards or some other cashless method.
Although the above-described metering technique works effectively for payphones connected to land lines, problems arise in attempting to implement the same technique for wireless payphones. With most wireless systems there is a need to minimize the amount of information transmitted from base stations to remote wireless telephones in order to maximize the capacity of the system. Avoiding transmission of redundant or uneccessary signaling information is one way of reducing the overall bandwidth consumed per call, and thereby maximizing system capacity. Dozens of wireless payphone calls may need to be transmitted more or less simultaneously within a wireless telephone system. The transmission of periodic metering pulses within each payphone call, particularly during peak hours, consumes a significant amount of bandwidth, which impacts the total number of calls that can be placed simultaneously. The problem is of particular concern within wireless systems employing digital transmission techniques such as code division multiple access (CDMA). For CDMA, the manner by which voice signals are processed makes it difficult to transmit voice signals with high frequency tones superimposed thereon. Accordingly, for a hypothetical wireless system to transmit metering pulses to a payphone, it may be necessary to convert the pulses to data packets, such as packets of the type employed in transmitting command and control signals, for interleaving with data packets containing digitized voice data. For a wireless local loop systems employing CDMA, in particular, constraints on the formatting of such packets could significantly affect the amount of bandwidth available for other signals such as commands and control signals and digitized voice signals. Hence, if additional data signals corresponding to metering pulses were transmitted throughout the entire duration of a telephone call to a wireless payphone, the amount of available bandwidth for the corresponding voice signals could be further reduced significantly, perhaps limiting the maximum possible quality of the transmitted voice signal. Such problems may be even more significant for wireless payphones mounted within aircraft, ships, or the like wherein voice quality may already be relatively limited by other transmission constraints. The need to process periodic metering signals throughout an entire telephone call may also impose a burden on the microprocessor, or other device, processing the various digital signals received from the remote station.
Accordingly, it would be desirable to provide an improved method and apparatus for performing payphone metering within a wireless telephone system incorporating one or more wireless payphones and it is to that end that aspects of the invention are primarily drawn.